The present invention relates generally to the protection of electrical apparatus and more particularly to a thermal model for an electrical apparatus from which can be determined representations of temperatures in the apparatus useful in protecting the apparatus from overtemperatures.
As is well known, excessive temperatures can occur in an electrical apparatus as a result of a sudden large overload, as a result of a small continuous overload, or as a result of some fault or interruption in the cooling or ventilating system. In addition, in three phase apparatus excessive temperature can arise from a phase loss or a phase unbalance condition. U.S. patent application Ser. No. 487,771, now U.S. Pat. No. 4,544,982, "Electrical Equipment Protection Apparatus and Method" by D. R. Boothman et al., filed on even date herewith and assigned to the assignee of the present invention describes a protection apparatus and method which makes use of a composite digital poly-phase current signal to protect against phase loss, phase unbalance, phase sequence reversal, and overtemperature. It includes a thermal model which determines temperatures and causes interruption of the electrical supply when predetermined temperatures are exceeded. The present invention is for an improved thermal model which can be used in a variety of situations including the protection apparatus of the aforementioned patent application. The thermal model of this invention can also be used to determine temperatures in single phase apparatus and to provide overtemperature protection for other electrical apparatus.
An electrical apparatus, such as a transformer or electric motor, normally has conductors having a relatively small thermal capacity and a core material having a larger thermal capacity. It is intended herein that the term "core material" be a general term; that is, it may include not only the iron core components but also other materials involved in the heat transfer such as insulation and supporting structure. Heat is generated in the conductors in accordance with the level of current and the conductor resistance and much of this heat is transferred to the core material at a rate which depends on the temperature difference between the conductors and the mass of the core material and the respective thermal resistivities. In addition, heat is normally dissipated from the core material at a rate which depends on the temperature difference between the core material and ambient (or any other cooling medium) and the thermal resistivity. Thus, there are several values for thermal capacity and thermal resistivity as well as the actual temperatures to be considered in any such model.
When the thermal analog or model is for a motor or other electrical apparatus with forced cooling, the same general situation exists. There is one rate of cooling when the motor is rotating and another when it is not rotating. In other electrical apparatus with forced cooling, such as fan cooling, there is one rate of cooling when the fan is operating and another when it is not operating.
It will be seen that there are a number of variables and a number of rates of heat transfer involved in a thermal model. One known way of making a very simple thermal model involves the use of one or more bimetallic elements with heaters responsive to current. This type of bimetallic element provides a time constant; that is, the heater heats the bimetallic element at a rate proportional to current and when the bimetal reaches a predetermined temperature it opens contacts to remove the supply of power to the apparatus it is protecting. If the current is below an acceptable value, a balance is reached between the heat generated in the heater and the heat loss so that the bimetallic element does become hot enough to open the contacts.
A more sophisticated thermal model is described in Canadian Pat. No. 983,094--Boothman et al., issued Feb. 3, 1976 to Canadian General Electric Company Limited. This described thermal model has a resistor-capacitor analog circuit simulating the thermal properties of the conductors, a charging circuit for charging the resistor-capacitor circuit at a rate proportional to current in the motor it is modelling, and a resistance circuit connected with the resistor-capacitor analog circuit simulating the thermal resistance of conductor insulation for discharging the resistance-capacitor circuit in accordance with heat transferred from the conductors. The voltage of the charge in the resistance-capacitance circuit represents temperature and is used to indicate temperatures over a predetermined limit or to trip a breaker to interrupt power to the motor.